We investigate the brightness evolution of seven FU Orionis
systems in the m wavelength range using data from the Infrared Space Observatory (ISO). The ISO measurements were
supplemented by 2MASS and MSX observations performed in the same
years as the ISO mission (1995-98). The spectral energy distributions
(SEDs) based on these data points were compared with earlier ones
derived from the IRAS photometry as well as from ground-based
observations carried out around the epoch 1983.
In three cases (Z CMa, Parsamian 21, V1331 Cyg) no difference between
the two epochs was seen within the measurement
uncertainties. V1057 Cyg, V1515 Cyg and V1735 Cyg have become
fainter at near-infrared wavelengths while V346 Nor has become
slightly brighter. V1057 Cyg exhibits a similar flux change in
the mid-infrared. At m most of the sources
remained constant; only V346 Nor seems to fade. Our data on the
long-term evolution of V1057 Cyg agree with the model predictions of
Kenyon & Hartmann ([CITE]) and Turner et al. ([CITE]) at near- and
mid-infrared wavelengths, but disagree at m.
We discuss if this observational result at far-infrared wavelengths
could be understood in the framework of the existing models.

Based on observations with ISO, an ESA project
with instruments funded by ESA member states (especially the PI
countries France, Germany, the Netherlands and the United Kingdom)
with participation of ISAS and NASA.

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